Ethanol traditionally has been produced from grain-based feedstocks (e.g., corn, sorghum/milo, barley, wheat, soybeans, etc.), or from sugar (e.g., sugar cane, sugar beets, etc.).
In a conventional ethanol plant, corn, sugar cane, other grain, beets, or other plants are used as a feedstock and ethanol is produced from starch contained within the corn, or other plant feedstock. In the case of a corn facility, corn kernels are cleaned and milled to prepare starch-containing material for processing. Corn kernels can also be fractionated to separate the starch-containing material (e.g., endosperm) from other matter (such as fiber and germ). Initial treatment of the feedstock varies by feedstock type. Generally, however, the starch and sugar contained in the plant material is extracted using a combination of mechanical and chemical means.
The starch-containing material is slurried with water and liquefied to facilitate saccharification, where the starch is converted into sugar (e.g., glucose), and fermentation, where the sugar is converted by an ethanologen (e.g., yeast) into ethanol. The fermentation product is beer, which comprises a liquid component, including ethanol, water, and soluble components, and a solids component, including unfermented particulate matter (among other things). The fermentation product is sent to a distillation system where the fermentation product is distilled and dehydrated into ethanol. The residual matter (e.g., whole stillage) comprises water, soluble components, oil, and unfermented solids (e.g., the solids component of the beer with substantially all ethanol removed, which can be dried into dried distillers grains (DDG) and sold, for example, as an animal feed product). Other co-products (e.g., syrup and oil contained in the syrup) can also be recovered from the whole stillage.
In a typical ethanol plant, a massive volume of whole stillage is generally produced. In fact, for a typical ethanol plant the amount of whole stillage produced can be nearly 13.4 gallons per bushel of corn processed. Roughly, a third of the corn feedstock is present in the whole stillage as dissolved and suspended solids. The stillage contains almost 90% water. Whole stillage is responsible for a substantial portion of the wastewater generated by ethanol plants. The financial cost of the water, its treatment and disposal (typically through evaporation) can be significant.
While stillage is typically viewed as a liability for an ethanol plant, it is possible to generate a number of high value co-products from the stillage. For example, oil in stillage, high protein feeds, and syrups are all able to be generated from stillage and sold as higher value co-products. Currently, in the interest of improving efficiencies of ethanol plants, whole stillage is often separated into two components: a solid component and a liquid component. Separation may be performed using centrifugation, or filter and press. The solid component may be dried to generate dried distillers grain (DDG) which is sold as animal feed. The liquid component, known as thin stillage, may be dried and used to increase the fat content of DDG to make DDGS (Distillers Dried Grains with Solubles). This process requires the drying of a large amount of water, which is very energy intensive and costly. Thin stillage may also be recycled into the plant, such as for replacement of some portion of the water used during fermentation (fermentation backset).
Further, there is currently a strong push to generate corn oil from stillage, as oil is a particularly high value commodity, and for regulatory and legal considerations. Ideally, an ethanol plant would generate a minimum of 1.33 pounds of oil product per bushel of corn processed. A number of oil recovery methods are known, but to date none has achieved more than about one pound of oil per bushel. Further, conventional processes for recovering oil from fermentation products often sacrifice oil quality such that the oil contains high levels of free fatty acids. The presence of a high level of free fatty acids can hamper the production of end products such as, for example, the yield and quality of any bio-diesel eventually produced with the oil as a feedstock.
For example, the patent application PCT/US2009/045163 (entitled “METHODS FOR PRODUCING A HIGH PROTEIN CORN MEAL FROM A WHOLE STILLAGE BYPRODUCT AND SYSTEM THEREFORE”) discloses a process for separating whole stillage into a solid portion and a thin stillage. The thin stillage is again separated to a protein and a water-soluble portion. An oil fraction may be separated from the water portion via evaporation. In the method of the PCT application, oil is recovered post evaporation, which likely results in yield reductions and oil quality sacrifices.
Another current method of oil recovery is disclosed by U.S. Pat. No. 7,829,680 (entitled “SYSTEM AND METHOD FOR ISOLATION OF GLUTEN AS A CO-PRODUCT OF ETHANOL PRODUCTION”). In this method, a plurality of screens is utilized to separate a fiber product from an oil/protein product. The oil and protein may then be separated via centrifugation.
While these known systems and methods may generate valuable co-products from ethanol production stillage, they have inherent drawbacks related to oil quality, quantity of oil recovered, water balance issues, and composition of the protein products isolated.